When a glass is broken due to impact, sharp glass fragments can cause injury to a human body, and a glass processed in order to prevent this is referred to as a safety glass in the broad sense. There are a laminated glass, a tempered glass, a wire plate glass, and the like, as the safety glass, and among them, a safety glass used in the narrow sense means a laminated glass, which is usually referred to as a laminated safety glass. The simplest structure of the laminated safety glass is that a laminated sheet is attached between two sheets of normal or tempered glass. According to the performances requested, multi-laminated safety glass, such as triple or quadruple laminated glass, may be used.
The laminated safety glass may be applied in various fields, such as a residential field including veranda window glass of high-rise apartments, partition glass of a bath room, and glass used in a door, a building field including skylights and the like, a vehicle field including front glass of car or train, and a safety or security field including bullet-proof glass through which bullets can not pass, an anti-theft glass of a store for managing valuables, an anti-riot glass of a prison, and the like.
The adhesive film or sheet for the laminated safety glass needs to satisfy high adhesive strength to glass, transparency, stability to sunlight and UV light, high tear strength, impact absorbing property, and the like.
Several kinds of material may be used as the adhesive sheet, but polyvinyl butyral (PVB) is the most commonly used in view of physical properties, performances, or manufacture. Beside this, ethylene vinyl acetate (EVA) or thermoplastic polyurethane (TPU) is partially being used.
A polyvinyl butyral (PVB) resin is obtained by butylating polyvinyl alcohol, and a butylation degree thereof is controlled at the time of adhering with glass, thereby allowing the PVB resin to have the required level of adhesive strength. However, the PVB resin is very tough and has high hardness. Therefore, even though the PVB resin is manufactured in a flat shape, the PVB resin is not easy to use at the time of manufacturing laminated glass. Furthermore, when the PVB resin is laminated on glass to manufacture laminated glass, penetration resistance that prevents an impact object from penetrating glass is difficult to secure. For this reason, addition of an appropriate level of plasticizer is needed to produce a film type having flexibility. Various kinds of additives may be used according to the application field, the requested performance, and the method for manufacturing the laminated glass, and thickness, adhesive strength, color, surface roughness, and the like may be differently exhibited depending on the kind of additive.
The laminated safety glass is obtained by putting an adhesive film between plate glasses after washing and drying of the adhesive film, putting the resultant the plate glasses with the adhesive film in an autoclave into which oil pressure or vapor pressure is applied, and pressing the resultant plate glasses with the adhesive film at 120 to 130° C. under about 15 kg/cm2.
A front glass of a car is usually referred to as a high penetration resistance (HPR) glass, for which a film having a thickness of 0.76 mm is mainly used. Here, it is important to make an adhesive film maintain an adhesive strength to glass at an appropriate level so that penetration resistance can be secured.
An adhesive film for building has a thickness of at least 0.38 mm, and may have a thickness of above 0.38 mm according to the performances and the needs in manufacturing the laminated glass.
The PVB film is being widely used due to excellent mechanical strength and adhesive strength thereof. However, the PVB film has a hydrophilic property, and thus, very sensitive to moisture. Therefore, when the film absorbs moisture, adhesive strength is deteriorated or there occurs color change or bubble generation. Therefore, since the PVB film or sheet needs to retain uniform humidity at a low temperature at the time of storage or transport, the PVB film or sheet needs to be separately water-proof packaged and needs to be stored or managed under appropriate temperature or humidity after unpackaging. In addition, in a case where bubble generation or binding failure occurs during working, the adhesive sheet and the glass are not easily separated from each other, and thus the entire glass should be discarded. In addition, when the laminated glass using PVB is severely exposed to moisture, a portion of the laminated glass may be delaminated, which prevents laminated glass from being used as a partition of the bathroom.